1. Field of the Invention
The present invention relates generally to seat belt restraint apparatus for restraining an occupant of an automobile, and more particularly relates to a seat belt retractor having a torsion bar with progressive load limiting characteristics.
2. Description of Related Art
Seat belt restraint systems for automobiles often include a locking mechanism, pretensioner, torsion bar, spool or spindle which is structured to lock and apply tension to the seat belt when an impact event such as an accident situation is detected. When the pretensioner is activated, the pretensioner eliminates any slack in the seat belt, and thus controls the physical space between the occupant and the seat belt. In this manner, the occupant of the seat is coupled with the seat belt as the occupant initially moves forward relative to the seat, thereby controllably restraining the occupant, reducing occupant excursion, and preventing undue loads when the occupant moves forwardly into the seat belt.
A retractor is another standard component of a seat belt restraint system which includes a spool receiving the webbing material of the seat belt, enabling retraction and extraction of the belt. The spool is used to wind up and store the webbing. Generally, the spool is locked in place upon detection at an impact situation in order to restrain the occupant via the seat belt. Recently, retractors have been designed having one or more force limiting elements which are structured to allow the spool to rotate and pay out the webbing material of the seat belt upon reaching predetermined belt tension force levels between the occupant and seat belt. In this manner, the restraint force imposed on the occupant can be limited in a controlled manner, thereby providing a certain load limitation characteristics.
One type of load limiting element for use with seat belt retractors is a torsion bar. A torsion bar is generally mounted between the spool or spindle of a belt retractor and a locking mechanism of a seat belt. In the event of a vehicle event causing a rapid deceleration, the locking mechanism can be actuated, either through the use of a controller that has sensed the event or through inertial forces. When the locking mechanism is actuated, the torsion bar and spindle attached thereto will be locked to prevent the seat belt from being extracted. The locked seat belt will cause a force to be exerted on the passenger by the belt, and the reaction force of the passenger against the belt will be transmitted back to the seat belt retractor. As force on the belt increases, the torsion bar will tend to twist, allowing the spindle to rotate slightly relative to the locking mechanism, thereby allowing a portion of the seat belt to be extracted. The extraction of the seat belt allowed by the twisting torsion bar will reduce the load exerted on the passenger by the belt.
A typical torsion bar acts to limit the load on the passenger in response to a single load level on the torsion bar being reached. To increase the number of load levels that can be reached where the load can be limited, one solution is to use multiple torsion bars mounted between the spindle and the locking mechanism. A first torsion bar can twist in response to a first load level exerted on the seatbelt, and a second torsion bar can subsequently twist in response to reaching a second load level. However, this solution can be undesirably costly to manufacture and time consuming to install.
Despite these and other improvements to automobile restraint systems, there remains a need to provide a retractor that includes a pretensioner while providing increased control and variation over the load limitation characteristics of the retractor system.